both StoredAsk and storage Provider are scoped to a single miner

[laser]

Why does this PR exist?

Passing a miner address to the getter doesn't make much sense, since the object itself is scoped to a single miner.

[codecov-commenter]

Codecov Report

Merging #276 into master will decrease coverage by 0.16%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
- Coverage   63.75%   63.60%   -0.15%     
==========================================
  Files          40       40              
  Lines        2416     2417       +1     
==========================================
- Hits         1540     1537       -3     
- Misses        752      756       +4     
  Partials      124      124              

Impacted Files	Coverage Δ
storagemarket/impl/provider.go	4.93% <0.00%> (-0.09%)	⬇️
storagemarket/types.go	33.34% <ø> (ø)
storagemarket/impl/storedask/storedask.go	80.00% <100.00%> (-0.51%)	⬇️
retrievalmarket/discovery/discovery.go	50.00% <0.00%> (ø)

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[hannahhoward]

I'm ok with this. I think the though was future versions might support multiple addresses? I'm not exactly sure why this would be but who knows.